firmware: arm_scmi: Fix device node validation for mailbox transport

commit 2ab4f4018cb6b8010ca5002c3bdc37783b5d28c2 upstream.

When mailboxes are used as a transport it is possible to setup the SCMI
transport layer, depending on the underlying channels configuration, to use
one or two mailboxes, associated, respectively, to one or two, distinct,
shared memory areas: any other combination should be treated as invalid.

Add more strict checking of SCMI mailbox transport device node descriptors.

Fixes: 5c8a47a5a91d ("firmware: arm_scmi: Make scmi core independent of the transport type")
Cc: <stable@vger.kernel.org> # 4.19
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20230307162324.891866-1-cristian.marussi@arm.com


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
[Cristian: backported to v4.19]
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>